The semiconductor industry continues to develop lithographic technologies, which can print ever-smaller integrated circuit dimensions. These systems must have high reliability, cost effective throughput, and reasonable process latitude. The integrated circuit fabrication industry has recently changed over from mercury G-line (436 nm) and I-line (365 nm) exposure sources to 248 nm and 193 nm excimer laser sources. This transition was precipitated by the need for higher lithographic resolution with minimum loss in depth-of-focus.
The demands of the integrated circuit industry will soon exceed the resolution capabilities of 193 nm exposure sources, thus creating a need for a reliable exposure source at a wavelength significantly shorter than 193 nm. An excimer line exists at 157 nm, but optical materials with sufficient transmission at this wavelength and sufficiently high optical quality are difficult to obtain. Therefore, all-reflective imaging systems may be required. An all reflective optical system requires a smaller numerical aperture than the transmissive systems. The loss in resolution caused by the smaller NA can only be made up by reducing the wavelength by a large factor. Thus, a light source in the range of 10 nm is required if the resolution of optical lithography is to be improved beyond that achieved with 193 nm or 157 nm.
The present state of the art in high energy ultraviolet and x-ray sources utilizes plasmas produced by bombarding various target materials with laser beams, electrons or other particles. Solid targets have been used, but the debris created by ablation of the solid target has detrimental effects on various components of a system intended for production line operation. A proposed solution to the debris problem is to use a frozen liquid or frozen gas target so that the debris will not plate out onto the optical equipment. However, none of these systems have proven to be practical for production line operation.
It has been well known for many years that x-rays and high energy ultraviolet radiation could be produced in a plasma pinch operation. In a plasma pinch an electric current is passed through a plasma in one of several possible configuration such that the magnetic field created by the flowing electric current accelerates the electrons and ions in the plasma into a tiny volume with sufficient energy to cause substantial stripping of outer electrons from the ions and a consequent production of x-rays and high energy ultraviolet radiation. Various prior art techniques for generation of high energy radiation from focusing or pinching plasmas are described in the following patents:
J. M. Dawson, xe2x80x9cX-Ray Generator,xe2x80x9d U.S. Pat. No. 3,961,197, Jun. 1, 1976.
T. G. Roberts, et. al., xe2x80x9cIntense, Energetic Electron Beam Assisted X-Ray Generator,xe2x80x9d U.S. Pat. No. 3,969,628, Jul. 13, 1976.
J. H. Lee, xe2x80x9cHypocycloidal Pinch Device,xe2x80x9d U.S. Pat. No. 4,042,848, Aug. 16, 1977.
L. Cartz, et. al., xe2x80x9cLaser Beam Plasma Pinch X-Ray System,xe2x80x9d U.S. Pat. No. 4,504,964, Mar. 12, 1985.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-Ray Apparatus,xe2x80x9d U.S. Pat. No. 4,536,884, Aug. 20, 1985.
S. Iwamatsu, xe2x80x9cX-Ray Source,xe2x80x9d U.S. Pat. No. 4,538,291, Aug. 27, 1985.
G. Herziger and W. Neff, xe2x80x9cApparatus for Generating a Source of Plasma with High Radiation Intensity in the X-ray Region,xe2x80x9d U.S. Pat. No. 4,596,030, Jun. 17, 1986.
A. Weiss, et. al, xe2x80x9cX-Ray Lithography System,xe2x80x9d U.S. Pat. No. 4,618,971, Oct. 21, 1986.
A. Weiss, et. al., xe2x80x9cPlasma Pinch X-ray Method,xe2x80x9d U.S. Pat. No. 4,633,492, Dec. 30, 1986.
I. Okada, Y. Saitoh, xe2x80x9cX-Ray Source and X-Ray Lithography Method,xe2x80x9d U.S. Pat. No. 4,635,282, Jan. 6, 1987.
R. P. Gupta, et. al., xe2x80x9cMultiple Vacuum Arc Derived Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,751,723, Jun. 14, 1988.
R. P. Gupta, et. al., xe2x80x9cGas Discharge Derived Annular Plasma Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 4,752,946, Jun. 21, 1988.
J. C. Riordan, J. S. Peariman, xe2x80x9cFilter Apparatus for use with an X-Ray Source,xe2x80x9d U.S. Pat. No. 4,837,794, Jun. 6, 1989.
W. Neff, et al., xe2x80x9cDevice for Generating X-radiation with a Plasma Sourcexe2x80x9d, U.S. Pat. No. 5,023,897, Jun. 11, 1991.
D. A. Hammer, D. H. Kalantar, xe2x80x9cMethod and Apparatus for Microlithography Using X-Pinch X-Ray Source,xe2x80x9d U.S. Pat. No. 5,102,776, Apr. 7, 1992.
M. W. McGeoch, xe2x80x9cPlasma X-Ray Source,xe2x80x9d U. S. Pat. No. 5,504,795, Apr. 2, 1996.
G. Schriever, et al., xe2x80x9cLaser-produced Lithium Plasma as a Narrow-band Extended Ultraviolet Radiation Source for Photoelectron Spectroscopyxe2x80x9d, Applied Optics, Vol. 37, No. 7, pp. 1243-1248, March 1998.
R. Lebert, et al., xe2x80x9cA Gas Discharged Based Radiation Source for EUV Lithographyxe2x80x9d, Int. Conf. On Micro and Nano Engineering, September, 1998.
W. T. Silfast, et al., xe2x80x9cHigh-power Plasma Discharge Source at 13.5 nm and 11.4 nm for EUV Lithographyxe2x80x9d, SPIE Proc. On Emerging Lithographic Technologies III, Vol. 3676, pp. 272-275, March 1999.
F. Wu, et al., xe2x80x9cThe Vacuum Spark and Spherical Pinch X-ray/EUV Point Sourcesxe2x80x9d, SPIE Proc. On Emerging Lithographic Technologies III, Vol. 3676, pp. 410-420, March 1999.
I. Fomenkov, W. Partlo, D. Birx, xe2x80x9cCharacterization of a 13.5 nm for EUV Lithography based on a Dense Plasma Focus and Lithium Emissionxe2x80x9d, Sematech International Workshop on EUV Lithography, October, 1999.
Typical prior art plasma focus devices can generate large amounts of radiation suitable for proximity x-ray lithography, but are limited in repetition rate due to large per pulse electrical energy requirements, and short lived internal components. The stored electrical energy requirements for these systems range from 1 kJ to 100 kJ. The repetition rates typically did not exceed a few pulses per second.
What is needed is a production line reliable, simple system for producing high energy ultraviolet and x-radiation which operates at high repetition rates and avoids prior art problems associated with debris formation.
The present invention provides a high energy photon source. A pair of plasma pinch electrodes are located in a vacuum chamber. The chamber contains a working gas which includes a noble buffer gas and an active gas chosen to provide a desired spectral line. A pulse power source provides electrical pulses at voltages high enough to create electrical discharges between the electrodes to produce very high temperature, high density plasma pinches in the working gas providing radiation at the spectral line of the source or active gas. Preferably the electrodes are configured co-axially. The central electrode is preferably hollow and the active gas is introduced out of the hollow electrode. This permits an optimization of the spectral line source and a separate optimization of the buffer gas. In preferred embodiments the central electrode is pulsed with a high negative electrical pulse so that the central electrode functions as a hollow cathode. Preferred embodiments present optimization of capacitance values, anode length and shape and preferred active gas delivery systems are disclosed. Preferred embodiments also include a pulse power system comprising a charging capacitor and a magnetic compression circuit comprising a pulse transformer. Special techniques are described for cooling the central electrode. In one example, water is circulated through the walls of the hollow electrode. In another example, a heat pipe cooling system is described for cooling the central electrode.
In preferred embodiments an external reflection radiation collector-director collects radiation produced in the plasma pinch and directs the radiation in a desired direction. Embodiments are described for producing a focused beam and a parallel beam. Also in preferred embodiments the active gas may be xenon or lithium vapor and the buffer gas is helium and the radiation-collector is made of or coated with a material possessing high grazing incidence reflectivity. Good choices for the reflector material are molybdenum, palladium, ruthenium, rhodium, gold or tungsten.
In other preferred embodiments the buffer gas is argon. Lithium vapor may be produced by vaporization of solid or liquid lithium located in a hole along the axis of the central electrode of a coaxial electrode configuration. Lithium may also be provided in solutions since alkali metals dissolve in amines. A lithium solution in ammonia (NH3) is a good candidate. Lithium may also be provided by a sputtering process in which pre-ionization discharges serves the double purpose of providing lithium vapor and also pre-ionization. In preferred embodiments, debris is collected on a conical nested debris collector having surfaces aligned with light rays extending out from the pinch site and directed toward the radiation collector-director. The reflection radiation collector-director and the conical nested debris collector could be fabricated together as one part or they could be separate parts aligned with each other and the pinch site.
This prototype devices built and test by Applicants convert electrical pulses (either positive or negative) of about 10 J of stored electrical energy per pulse into approximately 50 mJ of in-band 13.5 nm radiation emitted into 2xcfx80 steradians. Thus, these tests have demonstrated a conversion efficiency of about 0.5%, Applicants estimate that they can collect about 18 percent of the 50 mJ 13.5 nm radiation so that this demonstrated collected energy per pulse will be in excess of 7 mJ. Applicants have demonstrated 100 Hz continuous operation and 1000 Hz short burst operation. Thus, 0.7 Watt continuous and 7 Watt burst outputs have been demonstrated. Applicants have also shown that the techniques described herein can be applied to provide outputs in the range of 60 Watts at repetition rates of 5,000 Hz or greater. At 2000 Hz, the measured pulse-to-pulse energy stability, (standard deviation) was about 9.4% and no drop out pulses were observed. The electrical circuit and operation of this prototype DPF device is presented along with a description of several preferred modifications intended to improve stability, efficiency and performance.
The present invention provides a practical implementation of EUV lithography in a reliable, high brightness EUV light source with emission characteristics well matched to the reflection band of the Mo/Si or Mo/Be mirror systems. Tests by Applicants have demonstrated an improved electrode configuration in which the central electrode configuration in which the central electrode is hollow and configured as a cathode. For this configuration the hollow cathode produces its own pre-ionization so special pre-ionization is not needed.